1. Field of the Invention
The present invention concerns a method and device for evaluating the behavior of a given papermaking furnish in response to addition of chemical additives.
2. General Discussion of the Background
Paper makers have several simple methods and devices for predicting behavior of pulp on a paper machine. An example of such a method is estimating the drainage time of a papermaking stock. The drainage time can be estimated by the TAPPI Standard Drain Time, which requires placing a standardized sample of the stock in a small laboratory sheet mold and visually estimating the elapsed time until sheet formation after a drain valve is opened. The values so obtained may or may not correlate well with the performance of the same stock at the paper machine wet end depending on such variables as the pattern of chemical addition. Visual estimations of drainage time alone are subjective and only marginally capable of standardization.
A more sophisticated method for measuring the drainage characteristics of a papermaking stock is described in U.S. Pat. No. 4,613,406 to Gess. In this patent, a slurry on a screen is dewatered in a sheet mold using vacuum under a screen. The pressure differential across the screen is measured as a function of time as the sheet forms on the screen. When the data so obtained are plotted, the resulting curve shows four generally linear sections of different slopes. The first inflection point that occurs between the first two linear curves marks the transition point at which a random collection of fibers form a web as the stock is dewatered. This first inflection point is believed to correlate with the "wet line" on the forming section, which is the appearance of a flat, wet surface on the web. The second inflection point, which occurs between the second and third linear sections of the curve, indicates where the vacuum no longer compacts the web. This second inflection point is believed to correspond closely to the "dry line" on a forming section, which is the appearance of a dry surface on the web. Finally, a third inflection point occurs between the third and fourth linear portions of the curve, and indicates where the initial dewatering is essentially complete and air is first drawn completely through the web. The apparatus and method disclosed by Gess is described in Proceedings, TAPPI Papermaking Conference, Portland, Oreg., 185-189 (1983) and Notes, TAPPI Retention and Drainage Seminar, 75-81 (1983).
Additional information about the process of the Gess patent is provided by Gess in TAPPI, Advanced Topics in Wet End Chemistry Short Course, Memphis, Tenn. (1987). That paper analyzes a curve of sheet weight versus drainage time and discusses how this relationship predicts fines sensitivity of the forming system. The curve has two linear portions of differing slopes. The first linear portion of the plot is apparently related to the forming wire. In this portion of the plot, reaction is controlled by the pore size of the forming fabric, and the fines in the system pass through the pores of the forming fabric. The second linear portion is believed to reflect the fines retention characteristics of the sheet itself. The relationships between increasing sheet weight and drainage time are further analyzed by the same author in two more recent papers: TAPPI, Retention and Drainage Short Course, 49-52 (1989) and Preprints, TAPPI Annual Meeting, A27-A32 (1989).
A practical device that uses this relationship to predict performance of a stock on the forming section of a paper machine is described in U.S. Pat. No. 4,969,351. Sequential samples of furnish from a container are supplied to a sheet mold that automatically measures drainage time of each sheet formed from the furnish. The volume of each sequential sample of furnish is incremental such that a linear plot of drainage time versus increasing basis weight of the forming sheet is obtained. These representative data points allow an operator to predict drainage time of a sheet having any basis weight within the linear range of the data. The disclosed apparatus rapidly forms a series of sheets of differing basis weights to give the operator a timely indication of factors, such as white water fines buildup, that can affect operation of the forming section.
The device shown in U.S. Pat. No. 4,969,351 is unable, however, to monitor progress of physical or chemical changes in the slurry over time. The sequential aliquots of slurry must have uniform physical and chemical characteristics (be in equilibrium) for the linear plot of increasing basis weight accurately to predict drainage time. Hence, the properties of the slurry cannot be varied, and the sequential aliquots must be of different volumes.
Other patents have disclosed devices for determining the behavior of fiber pulp. U.S. Pat. No. 4,708,011, for example, weighs sequential pulp cakes on a wire screen and measures the pressure differential and volume of air flowing through the cake. U.S. Pat. No. 4,024,754 measures the rate of flow of a liquid forced under pressure through wood fiber pulp to assess the drainability of the material. Finally, U.S. Pat. No. 2,734,378 tests the filterability of a pulp mat on a screen by flowing water through the mat. In this patent, pulp is obtained at timed intervals from the refining end of a stock system and tested for filterability to provide an instantaneous running record of pulp freeness.
None of the aforementioned methods is able, however, to follow the effect of chemical additions of the elements of a papermaking furnish as a function of the time and mode of mixing the chemicals with the furnish. Prior systems also fail to provide a test system in which the reaction conditions of the test system mimic the kinetic conditions under which an additive is mixed with a pulp slurry in a papermaking line. Competing considerations of the free energy of reactants and collision kinetics are not addressed by the prior art systems that merely focus on the behavior of pulp at a specific point in time. Finally, some prior systems have the drawback of changing the physical and chemical characteristics of pulp slurry by unintentionally mixing the slurry with centrifugal pumps that transport the slurry within a test system.
It is therefore an object of the present invention to provide an improved method for evaluating a chemical reaction in a paper pulp furnish that accurately predicts behavior of the pulp on and in a paper machine system.
Another object of the present invention is to provide an improved device that follows the effect of chemical additives as a function of mixing time on a papermaking furnish to help select process parameters for producing cellulosic sheets from the slurry.
Another object is to provide such a method and device that can help evaluate competing reaction kinetics, such as the effects of the free energies of the products that can be formed versus the collision kinetics of a reaction.
Yet another object is to provide an improved tester that more accurately reproduces the kinetic conditions under which additives react with the slurry in a paper making machine, and can help choose preferred kinetic conditions for the reaction.
Finally, it is an object of the invention to provide an improved tester that transports slurry through the test system without substantially affecting the reaction kinetics in the pulp.
These and other objects of the invention will be understood more clearly by reference to the following description and drawings.